Membrane supported bioreactor for conversion of syngas components to liquid products

ABSTRACT

Ethanol and other liquid products are produced by contacting syngas components such as CO or a mixture of CO 2  and H 2  with a surface of a membrane and transferring these components in contact with a biofilm on the opposite side of the membrane. These steps provide a stable system for producing liquid products such as ethanol, butanol and other chemicals. The gas fed on the membrane&#39;s gas contact side transports through the membrane to form a biofilm of anaerobic microorganisms that converted the syngas to desired liquid products. The system can sustain production with a variety of microorganisms and membrane configurations.

RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. Provisional Patent Application 60/942,938 filed Jun. 8, 2007. The entirety of that application is incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to the biological conversion of CO and mixtures of CO₂ and H₂ to liquid products.

BACKGROUND

Biofuels production for use as liquid motor fuels or for blending with conventional gasoline or diesel motor fuels is increasing worldwide. Such biofuels include, for example, ethanol and n-butanol. One of the major drivers for biofuels is their derivation from renewable resources by fermentation and bioprocess technology. Conventionally, biofuels are made from readily fermentable carbohydrates such as sugars and starches. For example, the two primary agricultural crops that are used for conventional bioethanol production are sugarcane (Brazil and other tropical countries) and corn or maize (U.S. and other temperate countries). The availability of agricultural feedstocks that provide readily fermentable carbohydrates is limited because of competition with food and feed production, arable land usage, water availability, and other factors. Consequently, lignocellulosic feedstocks such as forest residues, trees from plantations, straws, grasses and other agricultural residues may become viable feedstocks for biofuel production. However, the very heterogeneous nature of lignocellulosic materials that enables them to provide the mechanical support structure of the plants and trees makes them inherently recalcitrant to bioconversion. Also, these materials predominantly contain three separate classes of components as building blocks: cellulose (C₆ sugar polymers), hemicellulose (various C₅ and C₆ sugar polymers), and lignin (aromatic and ether linked hetero polymers).

For example, breaking down these recalcitrant structures to provide fermentable sugars for bioconversion to ethanol typically requires pretreatment steps together with chemical/enzymatic hydrolysis. Furthermore, conventional yeasts are unable to ferment the C₅ sugars to ethanol and lignin components are completely unfermentable by such organisms. Often lignin accounts for 25 to 30% of the mass content and 35 to 45% of the chemical energy content of lignocellulosic biomass. For all of these reasons, processes based on a pretreatment/hydrolysis/fermentation path for conversion of lignocellulose biomass to ethanol, for example, are inherently difficult and often uneconomical multi-step and multi conversion processes.

An alternative technology path is to convert lignocellulosic biomass to syngas (also known as synthesis gas, primarily a mix of CO, H₂ and CO₂ with other components such as CH₄, N₂, NH₃, H₂S and other trace gases) and then ferment this gas with anaerobic microorganisms to produce biofuels such as ethanol, n-butanol or chemicals such as acetic acid, butyric acid and the like. This path can be inherently more efficient than the pretreatment/hydrolysis/fermentation path because the gasification step can convert all of the components to syngas with good efficiency (e.g., greater than 75%), and some strains of anaerobic microorganisms can convert syngas to ethanol, n-butanol or other chemicals with high (e.g., greater than 90% of theoretical) efficiency. Moreover, syngas can be made from many other carbonaceous feedstocks such as natural gas, reformed gas, peat, petroleum coke, coal, solid waste and land fill gas, making this a more universal technology path.

However, this technology path requires that the syngas components CO and H₂ be efficiently and economically dissolved in the aqueous medium and transferred to anaerobic microorganisms that convert them to the desired products. And very large quantities of these gases are required. For example, the theoretical equations for CO or H₂ to ethanol are:

6CO+3H₂O→C₂H₅OH+4CO₂

6H₂+2CO₂→C₂H₅OH+3H₂O

Thus 6 moles of relatively insoluble gases such as CO or H₂ have to transfer to an aqueous medium for each mole of ethanol. Other products such as acetic acid and n-butanol have similar large stoichiometric requirements for the gases.

Furthermore, the anaerobic microorganisms that bring about these bioconversions generate very little metabolic energy from these bioconversions. Consequently they grow very slowly and often continue the conversions during the non-growth phase of their life cycle to gain metabolic energy for their maintenance.

Many devices and equipment are used for gas transfer to micro organisms in fermentation and waste treatment applications. These numerous bioreactors all suffer from various drawbacks. In most of these conventional bioreactors and systems, agitators with specialized blades or configurations are used. In some others such as gas lift or fluidized beds, liquids or gases are circulated via contacting devices. The agitated vessels require a lot of mechanical power often in the range of 4 to 10 KW per 1000 gallons—uneconomical and unwieldy for large scale fermentations that will be required for such syngas bioconversions. The fluidized or fluid circulating systems cannot provide the required gas dissolution rates. Furthermore, most of these reactors or systems are configured for use with micro organisms in planktonic form i.e. they exist as individual cells in liquid medium.

Furthermore, to get high yields and production rates the cell concentrations in the bioreactor need to be high and this requires some form of cell recycle or retention. Conventionally, this is achieved by filtration of the fermentation broth through microporous or nonporous membranes, returning the cells and purging the excess. These systems are expensive and require extensive maintenance and cleaning of the membranes to maintain the fluxes and other performance parameters.

Cell retention by formation of biofilms is a very good and often inexpensive way to increase the density of micro organisms in bioreactors. This requires a solid matrix with large surface area for the cells to colonize and form a biofilm that contains the metabolizing cells in a matrix of biopolymers that the cells generate. Trickle bed and some fluidized bed bioreactors make use of biofilms to retain microbial cells on solid surfaces while providing dissolved gases in the liquid by flow past the solid matrix. They suffer from either being very large or unable to provide sufficient gas dissolution rates.

Particular forms of membranes have found use in supporting specific types microorganisms for waste water treatment processes. U.S. Pat. No. 4,181,604 discloses the use of hollow fiber membranes for waste treatment where the outer surface of the fibers supports a layer of microorganisms for aerobic digestion of sludge.

SUMMARY OF THE INVENTION

It has been found that contacting syngas components such as CO or a mixture of CO₂ and H₂ with a surface of a membrane and transferring these components in contact with a biofilm on the opposite side of the membrane will provide a stable system for producing liquid products such as ethanol, butanol and other chemicals. Accordingly this invention is a membrane supported bioreactor system for conversion of syngas components such as CO, CO₂ and H₂ to liquid fuels and chemicals by anaerobic microorganisms supported on the surface of membrane. The gas fed on the membrane's gas contact side transports through the membrane to a biofilm of the anaerobic microorganisms where it is converted to the desired liquid products

The instant invention uses microporous membranes or non-porous membranes or membranes having similar properties that transfer (dissolve) gases into liquids for delivering the components in the syngas directly to the cells that use the CO and H₂ in the gas and transform them into ethanol and other soluble products. The membranes concurrently serve as the support upon which the fermenting cells grow as a biofilm and are thus retained in a concentrated layer. The result is a highly efficient and economical transfer of the syngas at essentially 100% dissolution and utilization, overcoming limitations for the other fermentation methods and fermenter configurations. The syngas diffuses through the membrane from the gas side and into the biofilm where it is transformed by the microbes to the soluble product of interest. Liquid is passed in the liquid side of the membranes via pumping, stirring or similar means to remove the ethanol and other soluble products formed; the products are recovered via a variety of suitable methods.

A broad embodiment of this invention is a bioreactor system for converting a feed gas containing at least one of CO or a mixture of CO₂ and H₂ to a liquid product. The system comprises a bio-support membrane having a gas contacting side in contact with the feed gas for transferring said feed gas across the membrane to a biofilm support side for supporting a microorganism that produces a liquid product. The feed gas supply conduit delivers feed gas to the membrane system through a feed gas chamber having fluid communication with the gas supply conduit and the gas contact side of the membrane for supplying feed gas to said membrane. A liquid retention chamber in fluid communication with the biofilm support side of the membrane receives liquid products and a liquid recovery conduit in fluid communication with the liquid recovery chamber recovers a liquid product from the membrane system.

An additional embodiment of the instant invention includes the supply of dissolved syngas in the liquid phase to the side of the biofilm in contact with that phase. This allows dissolved gas substrate to penetrate from both sides of the biofilm and maintains the concentration within the biofilm at higher levels allowing improved reaction rates compared to just supplying the syngas via the membrane alone. This may be accomplished by pumping a liquid stream where the gases are predissolved into the liquid or by pumping a mixture of liquid containing the syngas present as small bubbles using fine bubble diffusers, jet diffusers or other similar equipment commonly used to transfer gas into liquids. The potential added advantage of using the combined gas and liquid stream is that the additional shear produced by the gas/liquid mixture may be beneficial in controlling the thickness of the biofilm. The advantage of pre-dissolution of the syngas is that very little, if any, of the gas is lost from the system so utilization efficiency is maximized.

Another embodiment of this invention includes the preferential removal of the carbon dioxide (CO₂) gas that is formed in the bioconversion process from the syngas using a membrane that selectively permeates CO₂ and then returning the syngas enriched in CO and H₂ to the bioreactor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing showing gas diffusing through a porous membrane into a liquid and details of a porous membrane, non-porous membrane and composite membrane.

FIG. 2 is a schematic drawing showing a central passage delivering gas to two parallel membrane walls with a liquid phase to the outside of each wall.

FIG. 3 is a schematic drawing showing the interior passage of FIG. 2 enclosed by the interior surface of the membrane in tubular form with liquid retained to around the membrane circumference.

FIG. 4 is a schematic drawing showing a bioreactor system with gas and liquid circulation.

FIG. 5 is a schematic drawing showing a bioreactor system with multiple bioreactors arranged in series having intermediate carbon dioxide removal.

DETAILED DESCRIPTION OF THE INVENTION

Bioconversions of CO and H₂/CO₂ to acetic acid, ethanol and other products are well known. For example, in a recent book concise description of biochemical pathways and energetics of such bioconversions have been summarized by Das, A. and L. G. Ljungdahl, Electron Transport System in Acetogens and by Drake, H. L. and K. Kusel, Diverse Physiologic Potential of Acetogens, appearing respectively as Chapters 14 and 13 of Biochemistry and Physiology of Anaerobic Bacteria, L. G. Ljungdahl eds, Springer (2003). Any suitable microorganisms that have the ability to convert the syngas components: CO, H₂, CO₂ individually or in combination with each other or with other components that are typically present in syngas may be utilized. Suitable microorganisms and/or growth conditions may include those disclosed in U.S. patent application Ser. No. 11/441,392, filed May 25, 2006, entitled “Indirect Or Direct Fermentation of Biomass to Fuel Alcohol,” which discloses a biologically pure culture of the microorganism Clostridium carboxidivorans having all of the identifying characteristics of ATCC no. BAA-624; and U.S. patent application Ser. No. 11/514,385 filed Aug. 31, 2006 entitled “Isolation and Characterization of Novel Clostridial Species,” which discloses a biologically pure culture of the microorganism Clostridium ragsdalei having all of the identifying characteristics of ATCC No. BAA-622; both of which are incorporated herein by reference in their entirety. Clostridium carboxidivorans may be used, for example, to ferment syngas to ethanol and/or n-butanol. Clostridium ragsdalei may be used, for example, to ferment syngas to ethanol.

Suitable microorganisms and growth conditions include the anaerobic bacteria Butyribacterium methylotrophicum, having the identifying characteristics of ATCC 33266 which can be adapted to CO and used and this will enable the production of n-butanol as well as butyric acid as taught in the references: “Evidence for Production of n-Butanol from Carbon Monoxide by Butyribacterium methylotrophicum,” Journal of Fermentation and Bioengineering, vol. 72, 1991, p. 58-60; “Production of butanol and ethanol from synthesis gas via fermentation,” FUEL, vol. 70, May 1991, p. 615-619. Other suitable microorganisms include Clostridium Ljungdahli, with strains having the identifying characteristics of ATCC 49587 (U.S. Pat. No. 5,173,429) and ATCC 55988 and 55989 (U.S. Pat. No. 6,136,577) and this will enable the production of ethanol as well as acetic acid. All of these references are incorporated herein in their entirety.

The instant invention uses microporous membranes or non-porous membranes or membranes having similar properties in being able to transfer (dissolve) gases into liquids for delivering the components in the syngas directly to the cells that use the CO and H₂ in the gas and transform them into ethanol and other soluble products. The membranes concurrently serve as the support upon which the fermenting cells grow as a biofilm and are thus retained in a concentrated layer. The result is a highly efficient and economical transfer of the syngas at essentially 100% dissolution and utilization, overcoming limitations for the other fermentation methods and fermenter configurations. The syngas diffuses through the membrane from the gas side and into the biofilm where it is transformed by the microbes to the soluble product of interest. Liquid is passed in the liquid side of the membranes via pumping, stirring or similar means to remove the ethanol and other soluble products formed; the products are recovered via a variety of suitable methods.

Microporous membranes made from polymers or ceramics have been recently developed and commercialized for wastewater treatment and purification applications. Some variations of these have also been developed for aeration or oxygenation of liquids. Typically these membranes are made from hydrophobic polymers such as polyethylene or polypropylene which are processed to create a fine porous structure in the polymer film. Many commercial organizations supply such membranes primarily in two important geometries—hollow fiber and flat sheets. These can then be made into modules by appropriate potting and fitting and these modules have very high surface area of pores in small volumes.

Suitable hydrophobic microporous hollow fiber membranes have been used for degassing applications to remove oxygen, carbon dioxide, and other gases from water and other liquids. An example of commercial membrane modules for such applications is the Liqui-Cel® membrane contactor from Membrana (Charlotte, N.C.), containing the polypropylene (PP) X40 or X50 hollow fibers. CELGARD® microporous PP hollow fiber membrane, containing the X30 fibers, is also available from Membrana for oxygenation applications. Liqui-Cel® membrane modules suitable for large scale industrial applications have large membrane surface areas (e.g., 220 m² active membrane surface area for Liqui-Cel® Industrial 14×28). Some characteristics of these fibers are given in the Table 1 below.

TABLE 1 X30 X40 X50 Porosity 40% 25% 40% (nominal) Pore Size 0.03 μm  0.04 μm  0.04 μm  Internal 240 μm 200 μm 220 μm Diameter Outer Diameter 300 μm 300 μm 300 μm Wall Thickness  30 μm  50 μm  40 μm

A microporous PP hollow fiber membrane product (CellGas® module) is available from Spectrum Laboratories (Rancho Dominguez, Calif.) for gentle oxygenation of bioreactors without excessive shear to the microbial or cell cultures. This PP hollow fiber is hydrophobic, with a nominal pore size of 0.05 μm and a fiber inner diameter of 0.2 mm.

For the use of hydrophobic microporous membranes for afore-mentioned applications, it is necessary to properly manage the pressure difference across the membrane to avoid formation of bubbles in the liquid. If the pressure difference is greater than a critical pressure, the value of which depends on properties of the liquid and the membrane, liquid can enter the pore (“wetting”) and the gas transfer rate is significantly impeded.

To prevent wetting of pores during operations, some composite membranes have been developed by the membrane suppliers. The SuperPhobic® membrane contactor from Membrana keeps the gas phase and liquid phase independent by placing a physical barrier in the form of a gas-permeable non-porous membrane layer on the membrane surface that contacts the process liquid. The SuperPhobic® 4×28 module contains 21.7 m² membrane surface area. Another composite hollow fiber membrane with an ultra-thin nonporous membrane sandwiched between two porous membranes is available from Mitsubishi Rayon (Model MHF3504) in the form of composite hollow fibers having at 34 m² membrane area per module.

Non-porous (dense) polymeric membranes have been used commercially for various gas separation applications. These membranes separate gases by the selective permeation across the membrane wall. The solubility in the membrane material and the rate of diffusion through the molecular free volume in the membrane wall determine its permeation rate for each gas. Gases that exhibit high solubility in the membranes and gasses that are small in molecular size permeate faster than larger, less soluble gases. Therefore, the desired gas separation is achieved by using membranes with suitable selectivity in conjunction with appropriate operating conditions. For example, Hydrogen Membranes from Medal (Newport, Del.) are used in recovery or purification of hydrogen with preferential permeation of hydrogen and CO₂. Medal also provides membranes for CO₂ removal with preferential permeation of CO₂.

Microporous membranes have been used widely in membrane bioreactors for wastewater treatment. Installations are mostly in the submerged membrane configuration using hollow fiber or flat sheet membranes for wastewater treatment. The structure and module configuration of these membranes may prove particularly useful for the systems of this invention. The membranes are typically made of poly(vinylidene fluoride) (PVDF), polyethylene (PE), PP, poly(vinyl chloride) (PVC), or other polymeric materials. The typical pore size is in the range of 0.03 to 0.4 μm. The typical hollow fiber outer diameter is 0.5 to 2.8 mm and inner diameter 0.3 to 1.2 mm. In these submerged membrane configurations, wastewater containing contaminants are fed into a tank and treated water is filtered through the membrane with a suction pressure applied to the filtrate side (the lumen side of the hollow fiber or the center of the flat plate) of the membrane. Typically the tank retains multiple membrane modules submerged without an individual housing. There are a number of commercial suppliers of membranes for submerged membrane bioreactors in wastewater treatment, each with some distinct features in membrane geometry and module design as described below. These membrane geometries and module designs can be suitable for the instant invention and are incorporated herein.

A hollow fiber membrane SteraporeSUN™, available from Mistubishi Rayon (Tokyo, Japan), is made of PE with modified hydrophilic membrane surface. The hollow fiber has a nominal pore size of 0.4 μm and a fiber outer diameter of 0.54 mm. A SteraporeSUN™ membrane unit Model SUN21034LAN has a total membrane surface area of 210 m², containing 70 membrane elements Model SUR334LA, each with 3 m² membrane area.

Another hollow fiber membrane SteraporeSADF™ is available from Mitsubishi Rayon. This membrane is made of PVDF with a nominal pore size of 0.4 μm and a fiber outer diameter of 2.8 mm. Each SteraporeSADF™ membrane element Model SADF2590 contains 25 m² membrane surface area, and each StreraporeSADF™ membrane unit Model SA50090APE06 containing 20 SADF2590 membrane elements has a total membrane surface area of 500 m².

Other commercial microporous hollow fiber membranes used for membrane bioreactors include but are not limited to the Zenon ZeeWeed® membranes from GE Water & Process Technologies (Oakville, Ontario, Canada), the Puron® membranes from Koch Membrane Systems (Wilmington, Mass.), and the MemJet® membranes from Siemens Water Technologies (Warrendale, Pa.).

Kubota Corporation (Tokyo, Japan) markets submerged membrane systems for membrane bioreactors. These membranes are of the flat-plate configuration and made of PVC with a pore size of 0.4 μm. Each membrane cartridge has 0.8 m² membrane surface area, and a Model EK-400 membrane unit, containing 400 membrane cartridges, has a total membrane area of 320 m².

Membranes of the various geometries and compositions described above may be used in arrangements of unitary arrays or assemblies of varied composition in the systems of this invention. Thus bio-support membrane used in the instant invention can be microporous, non-porous, or composite membranes or any combination thereof. Any suitable potting technique can be used to collect and provide the necessary assembly of individual membrane elements. If microporous, hydrophobic membranes are preferred due to faster diffusion of gases in the gas-filled pores than liquid-filled pores.

The feed gas flows through the gas chamber of the membrane unit continuously or intermittently. The feed gas pressure is in the range of 1 to 1000 psia, preferably 5 to 400 psia, and most preferably 10 to 200 psia. Operating at higher gas pressures has the advantage of increasing the solubilities of gases in the liquid and potentially increasing the rates of gas transfer and bioconversion. The differential pressure between the liquid and gas phases is managed in a manner that the membrane integrity is not compromised (e.g., the burst strength of the membrane is not exceeded) and the desired gas-liquid interface phase is maintained.

In such membranes the gas and liquid can be brought into direct and intimate contact without creating any bubbles by operating at a differential pressure that is below the bubble point of the membrane liquid interface and maintains the gas-liquid interface. Furthermore, the properties of this interface can be controlled by the porosity and hydrophobicity/hydrophlicity properties of the membrane pores.

In this invention, a bio-support membrane suitable for permeation of at least one of CO or a mixture of H₂ and CO₂ provides the separation between a feed gas and a liquid phase. FIG. 1 shows more detail of the membrane configuration and interface in the operation of a representative bio-reactor system. FIG. 1( a) depicts syngas stream A flowing to the gas feed side of the membrane in gas phase maintained in a chamber on the gas contact side of the membrane. The syngas components freely diffuse through the membrane pores to the liquid interface but without formation of bubbles. The anaerobic acetogenic bacteria, Clostridium ragsdaeli, having all of the identifying characteristics of ATCC No. BAA-622, is maintained in a fermentation media. The fermentation media is circulated through a chamber on the opposite side of the membrane that maintains a liquid volume in contact with the liquid side of the membrane. Suitable microbial cells are present as bio-film on the liquid-contacting side of the membrane surface, converting at least one of CO or H₂/CO₂ in the feed gas to desirable products. Since the membrane pores are much smaller than the width of the microorganisms they preferentially stay on the membrane surface to convert CO and H₂/CO₂ to gain metabolic energy, grow and form a biofilm on the membrane surface. A stream B withdraws the liquid phase components from a liquid volume retained about the outer surface of the biofilm.

FIGS. 1( b)-(c) show various forms of the membrane with a biofilm present on the liquid contacting side of the membrane. The membrane portions of FIGS. 1( a) and 1(b) both schematically show a cross-section of porous membrane to the left with a biofilm layer developed on the opposite side of the membrane. The interface between the biofilm and the membrane functions as equilibrium partitioning to keep the liquid and gas phases separated from each other. FIG. 1( c) depicts a similar arrangement however this time with a nonporous membrane to the left and a biofilm adhering to the surface on the right-hand side of the membrane. FIG. 1( d) illustrates a composite structure for the membrane that positions a porous membrane surface in contact with the gas phase components. The opposite face (right side) of the porous membrane retains a nonporous membrane layer and a biofilm layer adheres to the surface on the right side of the non-porous membrane layer.

FIG. 2 depicts a generalized view of a typical flow arrangement for efficient use of space in a membrane system. Syngas components enter the system as gas stream A and flow into a central space between two membrane walls. Gas phase contact surfaces of the opposing membrane walls form a distribution chamber for receiving gas from stream A. Gas permeates simultaneous through, in this case, the porous membrane for consumption by the microbes in the biofilm layers that adhere to the outer walls of the two opposing membranes. In this manner each gas channel serves multiple membrane surfaces and the stream B of liquid products is delivered from multiple membrane walls. The arrangement of FIG. 2 can use a flat sheet configuration and be particularly useful for good flow control and distribution on the liquid side that may be necessary for biofilm thickness control.

FIG. 3 shows the special case of FIG. 2 wherein the opposite wall of the central distribution chamber wrap around in continuous form to provide a tubular membrane. In this case gas stream A enters the lumen of the membrane and streams B of liquid products flow away from the outer walls in all directions. Hollow fibers are particularly useful for such bioreactor configuration.

FIG. 4 illustrates a specific configuration of one embodiment of this invention. A gas supply conduit delivers a feed gas Stream 10 containing CO, H₂, and CO₂ at a rate recorded by a flow meter 11. A feed gas distribution chamber 13 receives the feed gas stream and distributes the feed to the lumens of tubular membranes in a membrane unit 15 that provides a membrane supported bioreactor. A collection chamber 17 collects a portion of the feed gas that exits the lumens and an exhaust gas stream 12 from chamber 17 exits the membrane unit.

A tank surrounds the outside of the tubular membrane elements in the membrane supported bioreactor and retains a liquid for growth and maintenance of a biofilm layer on the outer surface of the membrane. The tank provides the means of temperature and pH controls for the liquid, which contains nutrients needed to sustain the activity of the microbial cells. The liquid in the tank is stirred to provide adequate mixing and sparged with a suitable gas, if necessary, to maintain a suitable gaseous environment. A re-circulating liquid loop, consisting of Streams 14, 16, and 18 re-circulates liquid through the tank. Liquid flows from the tank through lines 14 and 16 while line 20 withdraws liquid and takes to product recovery to recover liquid products. Line 18 returns the remaining liquid from line 16 to the tank via pump 19 at rate recorded by flow meter 21. The product recovery step removes the desirable product from Stream 20, while leaving substantial amounts of water and residual nutrients in the treated stream, part of which is returned to the bioreactor system via line 22. A nutrient feed is added via line 24 is added, as needed, to compensate for the amount of water removed and to replenish nutrients. Chamber 23 provides any mixing of the various streams and for return to the tank via line 18.

The flow rates of Streams 18 and 14, recirculated through the membrane unit, are selected so that there is no significant liquid boundary layer that impedes mass transfer near the liquid-facing side of the membrane and there is no excessive shear that may severely limit the attachment of cells and formation of the biofilm on the membrane surface. The superficial linear velocity of the liquid tangential to the membrane should be in the range of 0.01 to 20 cm/s, preferably 0.05 to 5 cm/s, and most preferably 0.2 to 1.0 cm/s. In addition to the liquid linear velocity, the biofilm thickness can be controlled by other means to create shear on the liquid-biofilm interface, including scouring of the external membrane surface with gas bubbles and free movement of the hollow fibers. Also, operating conditions that affect the metabolic activity of the microbial cells and the mass transfer rates of gases and nutrients can be manipulated to control the biofilm thickness. The biofilm thickness in the instant invention is in the range of 5-500 μm, preferably 5-200 μm.

Depending on the nature of the desired product, there are a number of technologies that can be used for product recovery. For example, distillation, dephlegmation, pervaporation and liquid-liquid extraction can be used for the recovery of ethanol and n-butanol, whereas electrodialysis and ion-exchange can be used for the recovery of acetate, butyrate, and other ionic products.

In all the depicted arrangement the CO an H₂ from the syngas are utilized and a gradient for their transport from the gas feed side is created due to biochemical reaction on the membrane liquid interface. This reaction creates liquid fuel or chemicals such as ethanol and acetic acid which diffuse into the liquid and are removed via circulation of the liquid past the biofilm. Thus the very large surface areas of the membrane pores are usable for gas transfer to the biofilm and the product is recovered from the liquid side. Furthermore, the reaction rate, gas concentration gradient and the thickness of the biofilm can be maintained in equilibrium because the microorganisms in the biofilm will maintain itself only up to the layer where the gas is available.

The membranes can be configured into typical modules as shown as an example in FIG. 4 for hollow fibers. The gas flows in the fine fibers that are bundled and potted inside a cylindrical shell or vessel through which the liquid is distributed and circulated. Very high surface areas in the range of 1000 m2 to 5000 m2 per m3 can be achieved in such modules.

The bioreactor modules can be operated multi-stage operation of fermentation using the modules in counter-current, co-current or a combination thereof mode between the gas and the liquid. In the example as shown in FIG. 4 a counter current operation is depicted.

During the bioconversion excess CO₂ is generated and this gas can diffuse back and dilute out the concentrations of CO and H₂ in the feed gas and thus reduce their mass transfer rates. Other types of membranes that preferentially permeate CO₂ over CO and H₂ can be used in the multi stage configuration as shown as an example in FIG. 5 where, using a membrane that selectively permeates CO₂ and then returning the syngas enriched in CO and H₂ to the bioreactor can be achieved.

FIG. 5 depicts a system where the entering feed gas flows into bioreactor 27 via line 26 and serially through bioreactors 29 and 31 via lines 28, 32 and 34. At the same time liquid that contacts the biofilm layers enters the system via line 38 and flows countercurrently, with respect to the gas flow, through bioreactors 31, 29 and 27 via lines 40 and 42. Liquid products are recovered from the liquid flowing out of line 40 and gas stream is withdrawn from the system via line 36. Separation unit 33 provides the stream of line 28 with intermediate removal of CO₂ from the system via any suitable device or process such as a membrane or extraction step. Interconnecting lines 40 and 42 also provide the function of establishing continuous communication through all of the lumens of the different bioreactors so that any combined collection and distribution chambers provide a continuous flow path.

Other microorganisms can also be used in the examples and configurations described above. The anaerobic acetogenic bacteria, Clostridium carboxidivorans having all of the identifying characteristics of ATCC no. BAA-624; can be used and this will enable the production of ethanol, n-butanol and acetic acid.

Another anaerobic bacteria Butyribacterium methylotrophicum, having the identifying characteristics of ATCC 33266 can be adapted to CO and used and this will enable the production of n-butanol as well as butyric acid.

Another anaerobic bacteria Clostridium Ljungdahlii, having the identifying characteristics of ATCC 55988 and 55989 can be used and this will enable the production of ethanol as well as acetic acid.

EXAMPLE

A Liqui-Cel® membrane contactor MiniModule® 1×5.5 from Membrana (Charlotte, N.C.) is used as a membrane supported bioreactor for the conversion of carbon monoxide and hydrogen into ethanol. This membrane module contains X50 microporous hydrophobic polypropylene hollow fibers with 40% porosity and 0.04 μm pore size. The fiber outer diameter is 300 μm and internal diameter 220 μm. The active membrane surface area of the module is 0.18 m². A gas containing 40% CO, 30% H₂, and 30% CO₂ is fed to the lumen of the fibers at 60 std ml/min and 2 psig inlet pressure and the residual gas exits the module at 1 psig outlet pressure. The membrane module is connected to a 3-liter BioFlo® 110 Fermentor from New Brunswick Scientific (Edison, N.J.). The fermentation medium having the composition given in Table 2 is pumped from the fermentor, flows through the shell side of the membrane module, and returns to the fermentor. The flow rate of this recirculating medium is 180 ml/min, and the pressure at the outlet of the membrane module is maintained at 5 psig by adjusting a back-pressure valve. The fermentor contains 2 liters of the fermentation medium, which is agitated at 100 rpm and maintained at 37° c. The fermentor is maintained under anaerobic conditions.

The fresh fermentation medium contains the components listed in Tables 2 & 3(a)-(d). Initially, the bioreactor system is operated in the batch mode and inoculated with 200 ml of an active culture of Clostridium ragsdalei ATCC No. BAA-622. The fermentation pH is controlled at pH 5.9 in the first 24 hours by addition of 1 N NaHCO₃ to favor cell growth and then allowed to drop without control until it reaches pH 4.5 to favor ethanol production. The system remains in the batch mode for 10 days to establish the attachment of the microbial cells on the membrane surface. Then, the system is switched to continuous operation, with continuous withdrawal of the fermentation broth for product recovery and replenish of fresh medium. With the continuous operation, suspended cells in the fermentation broth are gradually removed from the bioreactor system and decrease in concentration, while the biofilm attached on the membrane surface continues to grow until the biofilm reaches a thickness equilibrated with the operating conditions. The ethanol concentration at the end of the 10-day batch operation is 5 g/L. At the beginning of the continuous operation, a low broth withdrawal rate is selected so that the ethanol concentration in the broth does not decrease but increases with time. The broth withdrawal rate is then gradually increased. After 20 days of continuous operation, the ethanol concentration increases to 10 g/L with the broth withdrawal rate at 20 ml/hr.

TABLE 2 Fermentation Medium Compositions Amount Components per liter Mineral solution, See Table 2(a) 25 ml Trace metal solution, See Table 2(b) 10 ml Vitamins solution, See Table 2(c) 10 ml Yeast Extract 0.5 g Adjust pH with NaOH 6.1 Reducing agent, See Table 2(d) 2.5 ml

TABLE 3(a) Mineral Solution Components Concentration (g/L) NaCl 80 NH₄Cl 100 KCl 10 KH₂PO₄ 10 MgSO₄•7H₂O 20 CaCl₂•2H₂O 4

TABLE 3(b) Trace Metals Solution Components Concentration (g/L) Nitrilotriacetic acid 2.0 Adjust the pH to 6.0 with KOH MnSO₄•H₂O 1.0 Fe(NH₄)₂(SO₄)₂•6H₂O 0.8 CoCl₂•6H₂O 0.2 ZnSO₄•7H₂O 1.0 NiCl₂•6H₂O 0.2 Na₂MoO₄•2H₂O 0.02 Na₂SeO₄ 0.1 Na₂WO₄ 0.2

TABLE 3(c) Vitamin Solution Concentration Components (mg/L) Pyridoxine•HCl 10 Thiamine•HCl 5 Roboflavin 5 Calcium Pantothenate 5 Thioctic acid 5 p-Aminobenzoic acid 5 Nicotinic acid 5 Vitamin B12 5 Mercaptoethanesulfonic acid 5 Biotin 2 Folic acid 2

TABLE 3(d) Reducing Agent Components Concentration (g/L) Cysteine (free base) 40 Na₂S•9H₂O 40 

1. A bioreactor system for converting a feed gas containing at least one of CO or a mixture of CO₂ and H₂ to a liquid product comprising: a) a bio-support membrane having a gas contacting side in contact with the feed gas for transferring said feed gas across the membrane to a biofilm support side for supporting a microorganism that produces a liquid product; b) a feed gas supply conduit for delivering feed gas to the membrane system; c) a feed gas chamber in fluid communication with the gas supply conduit and the gas contact side of the membrane for supplying feed gas to said membrane; d) a liquid retention chamber in fluid communication with the biofilm support side of the membrane for receiving liquid products; and, e) a liquid recovery conduit in fluid communication with the liquid recovery chamber for recovering a liquid product from the membrane system.
 2. The system of claim 1 wherein the bio-support membranes comprises a micro porous membrane and/or a non-porous membrane.
 3. The system of claim 1 wherein the microorganism produces a liquid product comprising at least one of ethanol, n-butanol, acetic acid, and butyric acid.
 4. The system of claim 1 wherein the feed gas is synthesis gas, the support side of the membrane supports a microorganism that produces ethanol and the liquid recovery conduit recovers an ethanol containing liquid.
 5. The system of claim 1 wherein the liquid retention chamber contains one or more dissolved gases for contact with the biofilm and the dissolved gases include at least one of CO and CO₂ and H₂.
 6. The system of claim 5 wherein the dissolved gas comprises synthesis gas that enters the liquid retention chamber in solution with a liquid stream or as small bubbles.
 7. The system of claim 1 wherein the liquid support chamber agitates liquid within the chamber to provide shear forces to control the thickness of the biofilm.
 8. The system of claim 1 wherein the feed gas passes serially through multiple bio-support membranes, the system includes at least one feed gas chamber for each bio-support membrane and CO₂ is removed from the feed gas as it passes through the system.
 9. The system of claim 8 wherein the liquid product passes serially through at least one liquid retention chamber for each bio-support membrane, a feed gas is withdrawn from each feed gas chamber, and the feed gas and liquid products pass in co-current flow, countercurrent flow or a combination thereof.
 10. The system of claim 1 wherein the bio-support membrane is hydrophobic.
 11. The system of claim 1 wherein the bio-support membrane comprises a plurality of hollow fiber membranes and the feed gas chamber includes the collective lumen volume of the fibers.
 12. The system of claim 11 wherein the liquid chamber includes hollow fibers membranes for removing dissolved CO₂ from the liquid phase.
 13. The system of claim 1 wherein the microorganism supported by the bio-support membrane comprises a mono-culture or a co-culture of at least one of Clostridium ragsdalei, Butyribacterium methylotrophicum, and Clostridium Ljungdahlii.
 14. The system of claim 1 wherein a continuous flow of feed gas passes across the gas contact side of the bio-support membrane.
 15. A bioreactor system for converting a synthesis gas to a liquid product comprising: a) a gas supply conduit for delivering synthesis gas; b) a distribution chamber in fluid communication with the supply conduit; c) a plurality of hollow fiber membranes having a first lumen end in fluid communication with the distribution chamber and outer surface suitable for the supporting a biofilm comprising microorganisms for producing liquid products from the synthesis gas; d) a liquid retention chamber surrounding at least a portion of the plurality of hollow fiber membranes and providing agitation to the outer surface of the hollow fibers to control the thickness of the biofilm; e) a liquid recovery conduit in fluid communication with the liquid recovery chamber for recovering liquid products.
 16. The system of claim 15 wherein the hollow fiber membranes comprise a micro porous membrane and/or a non-porous membrane.
 17. The system of claim 15 wherein the microorganism produces a liquid product comprising at least one of ethanol, n-butanol, acetic acid, and butyric acid.
 18. The system of claim 15 wherein the feed gas is synthesis gas, the support side of the membrane supports a microorganism that produces ethanol and the liquid recovery conduit recovers an ethanol containing liquid.
 19. The system of claim 15 wherein the liquid retention chamber contains one or more dissolved gases comprising one or more components of the synthesis gas that enter the liquid retention chamber in solution with a liquid stream or as small bubbles.
 20. A bioreactor system for converting a synthesis gas to a liquid product comprising: a) a fiber membrane bundle comprising hollow fiber membranes having first and second lumen ends and an outer surface suitable for the supporting a biofilm comprising microorganisms for producing ethanol from the synthesis gas; b) a gas supply conduit and gas for delivering a continuous stream of synthesis gas; c) a distribution chamber in fluid communication with the supply conduit and the first lumen ends; d) a collection chamber in fluid communication with the second lumen end; e) gas recovery conduit in fluid communication with the collection chamber for continuously withdrawing synthesis gas; f) a liquid retention chamber surrounding at least a portion of the plurality of hollow fiber membranes and providing agitation to the outer surface of the hollow fibers to control the thickness of the biofilm; and, g) a liquid supply conduit and a liquid recovery conduit in fluid communication with the liquid retention chamber for circulating liquid through the liquid retention chamber and recovering an ethanol containing liquid from the liquid retention chamber.
 21. The system of claim 20 wherein the feed gas passes serially through multiple bio-support membranes, the system includes at least one distribution chamber and at least one collection chamber for each bio-support membrane, at least one collection chamber transfers synthesis directly to at least one distribution chamber, and CO₂ is removed from the feed gas as it passes through the system. 